Process for dry scrubbing of flue gas

ABSTRACT

A flue gas dry scrubbing process and system achieves higher sulfur dioxide removal efficiency with a better utilization of alkali reagent chemicals as well as increased dry product recovery performance using a combination of spray drying with an alkali reagent slurry and with a recycled stream comprising a mixture of unreacted alkali, sulfite and sulfate reaction products and flyash which directly contacts the sulfur dioxide-containing flue gas. Preferably the recycled mixture contacts the flue gas in the spray dryer. The invention utilizes a splitting method and a splitter apparatus wherein the particle product powder is separated into two fractions for recycling and for product disposal. The rate of splitting is fully adjustable over a wide range.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the removal of sulfur dioxide and particulatematerial from boiler flue gases. In particular, this invention relatesto a process and system for dry scrubbing desulfurization of flue gasesusing soda ash, lime, or similar alkali reagents.

2. Description of the Prior Art

The wet scrubbing of boiler flue gases with alkali slurries to removesulfur dioxide is well known. Although wet scrubbers are effective andhave enjoyed wide application, a drawback associated with their use isthe difficulty of waste product disposal. That is, the common by-productof the desulfurization of flue gases by wet scrubbing is sludge whichmay contain up to 50 weight percent water. Sludge requires comparativelyexpensive disposal means, such as on site settling ponds or costlytransportation to remote disposal sites, as well as some stabilizationexpenses.

In order to overcome this disadvantage of wet scrubbing, it has beenproposed to utilize a dry product system in which an atomized solutionor slurry of alkali reactant is mixed with flue gas in a spray dryer sothat the sulfur dioxide contained in the flue gas is absorbed on thesurface of fine drops in the atomized spray and the alkali materialreacts with the sulfur dioxide to form sulfite and sulfate compounds. Adry powder mixture of these compounds, with residual unreacted alkali,is formed as a result of the thermal energy of the flue gas. The fluegas, containing a particulate mixture of the reaction products,unreacted alkali and any flyash originally present, exits the spraydryer and is transported to dust removal equipment such as a fabricfilter or electrostatic precipitator. The cleaned gas is then exhaustedand the particulates are removed from the dust collector hoppers in adry form for disposal.

It is stated that advantages of dry scrubbers over wet scrubbers includesimplicity of operation, lower capital costs for systems, smaller energydemands of operation as well as easier transportation and handling ofdry wastes for end product disposal. Environmental requirements may alsobe more readily met by such systems.

In the articles "Two-State Dry Scrubbers Come of Age Interest Booms,Orders Start Coming In", Electric Light and Power, September, 1978, pp.49-50, and "Tests of a Two-Stage Combined Dry Scrubber/SO₂ AbsorberUsing Sodium or Calcium", Combustion, November 1978, pp. 30-43, a dryscrubbing system is described. In that system, boiler flue gas enters aspray dryer where a mist with a dilute reagent solution or slurry issprayed through the gas. Quick chemical reaction is said to remove asubstantial portion of the sulfur dioxide from the flue gas while theheat in the gas evaporates the water and dries the solution to form adry powder. Flue gas containing the dry powder and flyash leaves thespray dryer and enters either a precipitator or fabric filter drycollector where, it is said, additional reaction of the sulfur dioxidetakes place as the flyash and powder are removed from the gas in thecollector. The clean gas is then exhausted to a stack while flyash andsulfur dioxide-containing powder particulates are removed from thecollector hoppers in dry form for disposal. Among the dry solidsrecovered are the reagent materials, such as lime, which may be recycledas absorbent feed to the slurry feed tank to be mixed with make-up limeslurry before being atomized in the spray dryer for further use.

The prior art dry scrubbing systems are currently essentially still inthe pilot testing stage. In Fabric Filter Newsletter, Nov. 10, 1978; No.37, pp. 3-4, it is stated that 90 percent sulfur dioxide collection maybe achieved using either soda ash or lime as a reagent. With soda ash,this level of sulfur dioxide removal is said to be obtained with astoichiometric ratio of 1.0 to 1.2 while using lime as an absorbentrequired a stoichiometry of between 2.3 and 3.0 for the same removalefficiency. With recirculation of dried and partially reacted productand flyash from the spray dryer and filter bag house to the feed tankfor mixing with make-up lime slurry before being again atomized, limeutilization is said to be increased with a 90 percent removal efficiencyallegedly being obtained with a stoichiometry in the range of 1.3 to1.7.

Although the prior art dry scrubbing systems present the inherentadvantages that dry scrubbing provides over wet scrubbing, theynevertheless incur high reagent costs due to the high stoichiometricratios of reagent to sulfur dioxide required to obtain the desiredremoval efficiency. For example, lime removal efficiencies of 90 percentare attained only at undesirably low utilization efficiences. This is aparticular drawback where sulfur contents in coal are high, such asabove about 1.4 percent. Also, where filter bag house, or other dustcollector, discharge powder is recycled to the feed tank, the amount ofrecycle is severely limited by the amount of water which can be used,which is determined by the inlet and outlet temperatures of the spraydryer, and the upper limit of slurry concentration which the spray dryeratomizer can accept.

Thus, there exists a need for dry scrubbing processes and systems whichpossess increased efficiencies and result in reduced reactant costs and,concurrently, decreased waste amounts and reduced disposal costs.

SUMMARY OF THE INVENTION

The present invention provides an improvement over the dry scrubbingprocesses and systems now known by achieving higher sulfur dioxideremoval efficiency with an improved stoichiometry and thus a betterutilization of alkali reagent chemicals as well as increased dry productrecovery performance. This is achieved, according to the invention, in aprocess and system using a combination of spray drying with an alkalireagent slurry and dry scrubbing with a recycled dry powder streamcomprising a mixture of unreacted alkali, sulfite and sulfate reactionproducts and flyash. The recycled stream is recycled as a powdersuspension in the flue gas and is injected into the top of the spraydrying chamber until it falls virtually downwards through the spray zoneproduced by the atomizer.

Accordingly, a feature of this invention is the provision of a processand system for the removal of sulfur dioxide from flue gases using spraydrying and dry scrubbing with recycling.

Several advantages result from the process and system of this invention.These include a higher sulfur dioxide removal efficiency and improvedstoichiometry than that obtainable with the known methods. In addition,the amount of recycled powder can be adjusted and the same alkali causedto pass through the spray dryer many times with greatly increasedexposure to sulfur dioxide in the flue gas. Thus, the alkali reagent,such as lime, is better utilized so that there is a lower consumption ofthe chemicals as a consequence of the increased reagent total exposuretime. Furthermore, by conveying the recycled powder mixture atrelatively high velocity in the conveying pipes, there is a furtheradvantage of a continuous creation of fresh surface area as aconsequence of powder fracture and alkali particle attrition and hencean additional lowering of chemical consumption. Another advantage isthat most flyash contains alkali such as CaO, MgO, Al₂ O₃, Na₂ O, K₂ O,and the like. By recycling a large amount of flyash, these alkali willreact with sulfur dioxide and, thereby, also reduce the required feed ofalkali reagent chemical.

The process and system of this invention have the inherent advantagethat temperature control becomes less critical. That is, since a largeamount of powder is recycled, the amount of water into the atomizer canbe reduced, thereby operating at a higher outlet temperature. This willreduce the chance of making the filter bag in the dust collectionbaghouse wet. Also, by introducing the recycled dry powder mixture intothe slurry droplet zone at the spray dryer, the powder will have adrying effect and the chance of wetting the walls in the spray dryerchamber, and thereby causing particle buildup on the walls, will besignificantly reduced.

A further feature of this invention is the provision of a process andsystem for dry scrubbing of flue gases with spray drying wherein reactedand unreacted reagent alkali and flyash are recycled and directlycontact, in the dry state, sulfur dioxide containing gas in the spraydryer chamber.

The process and system of this invention utilize a splitting method anda splitter apparatus wherein the particulate product powder is separatedinto fractions one of which is recycled and one of which proceeds toproduct disposal. The rate of splitting is fully adjustable over a widerange.

Thus, another feature of the invention is the provision of a method andapparatus for separating part of the dry product into a recycle streamand a disposal stream.

The foregoing and other features, advantages and objects of thisinvention will be further apparent from the following description of thepreferred embodiments herein taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration, in schematic form, of an overall system fordry scrubbing of flue gas according to an embodiment of this invention;

FIG. 2 is a rear elevational view of a splitter apparatus for the systemaccording to an embodiment of this invention; and

FIG. 3 is a side view taken along the line 3--3. with a portion brokenaway for greater clarity, of the splitter apparatus of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, there is shown a system for dry scrubbing of fluegas according to a preferred embodiment of this invention. Flue gas froma boiler enters the system through a duct 10 and is transported to aspray dryer 12 where it is contacted with unreacted and recycled alkali.The alkali, in this instance lime, is fed to the spray dryer 12 from therecycle silo 14 and the lime slurry preparation tank 16 through the feedlines 18 and 20, respectively.

Fresh lime slurry from the lime slurry preparation tank 16 is pumped tothe spray dryer 12 through the pipe 20 to an atomizing nozzle 22 andinto a spray drying chamber 24. Steam from the boiler also enters theatomizing nozzle 22, through the pipe 26, and breaks up the lime slurryinto small droplets. The rate of fresh lime slurry feed is controlled bya valve 28 which is controlled by a temperature controller 30 whichsenses temperature in the duct 32 exiting the spray dryer 12.

Fresh lime slurry is prepared in the lime slurry preparation tank 16 byfeeding lime from the lime storage silo 34 through the metering limefeed screw conveyor 36 to the top of the lime slurry preparation tank16. Water is added to the dry lime powder through the pipe 38 and thetwo mixed by means of the stirrer 40.

Sulfur dioxide in the flue gas is absorbed on the surface of the limeslurry droplets which form as a dispersed spray of fine alkali slurrycontaining droplets as a result of the atomizing nozzle 22. The sulfurdioxide reacts with the lime in the droplets, in the spray dryingchamber 24, to form calcium sulfites and calcium sulfates. At the sametime, the heat of the flue gas evaporates the water in the droplets toproduce a dry powder mixture of those reaction products and of unreactedlime. As the slurry droplet water evaporates the flue gas temperature isreduced.

The flue gas, which now contains a mixture of particulate solidsincluding the reaction products, unreacted lime, and flyash which mayremain present, exits the spray dryer 12 through the duct 32 and istransported to a dust collection system referred to generally at 42.Some of the particulate solids fall out in the hopper 84 of the spraydrying vessel 12 and some in the hopper near the point where the fluegas enters the spray drying chamber 24, shown at 44, and collect in thebottom of the spray drying vessel 12 as flyash and other solids fromwhence they are removed through the discharge lines 46 and 88 to enterthe solids conveying line 48.

The flue gas and particulate solids stream leaves the spray dryer 12through the duct 32 and enters the dust collection system 42 where amanifold 49 distributes the steam to fabric filters 50 through inletconduits 52. The filtered flue gas exits the filter through the conduits54, enters the outlet manifold 56 and passes to the main ID fan 58 andfrom there to the stack 60 where it is exhausted as clean flue gas tothe atmosphere. The particulate solids removed by the fabric filters 50are collected and are discharged through the discharge lines 62 whichfeed them to the solids conveying line 58.

The particulate solids, or dust, collected on the fabric filters 50, inthis instance filter bags, are removed by reversing the gas flow throughthe filter bags with the help of a blower 64 and reverse ducting 66utilizing the cleaned flue gas. The powder dust falls into the hoppersof filter 68 and is removed by rotary valves 70 to the discharge lines62.

The powder dust from the collection system 42 is conveyed through theline 48 to the storage silo 14. The gas for the conveying is cleaned hotflue gas which is extracted from the outlet duct 56 of the dustcollection system 42 and is split into a stream, 72, for conveying thesolids and a stream, 74, for reverse gas flow through the filter bags50. The use of hot flue gas helps prevent water condensation in the line48 and ducts 66.

The solids conveying line 48, which also receives solid particulatesfrom the spray dryer 12 through the discharge lines 46 and 88, conveysmaterial to the top of the storage silo 14. The silo 14 is ventilatedwith a small dust collector 74 and a fan 76 which further transports thehot conveying gas downwards in the duct 18. A splitter apparatus 78 islocated at the bottom of the storage silo 14.

The splitter apparatus 78, described in greater detail below withreference to FIGS. 2 and 3, divides the solid particulates in thestorage silo 14 into two streams, one of which is injected as recyclepowder into the duct 18 and the other of which is removed from thesystem by the rotary valve 80 and the waste conduit 82 for disposal.

The recycled powder injected into the duct 18 is conveyed by theconveying gas to the top of the spray dryer 12 where it is injected downinto the zone of the spray drying chamber 24 by injector pipes 84located such that the recycled powder is well distributed across thespray drying chamber. The recycled powder falls downward through thespray drying chamber 24 where it is contacted by the flue gas andfurther sulfur dioxide within the flue gas is absorbed on the unreactedalkali within the recycled powder. A part of the recycled powder willfall into the hopper portion 84 of the spray dryer 12 to be removedtherefrom through the rotary valve 86 and the discharge line 88 for feedinto the solids conveying line 48. The remaining part of the recycledpowder exits the spray dryer with the flue gas through the conduit 32and is removed therefrom in the dust collection system 42.

The flue gas, pressurized by fan 76, conveys the recycled powder fromthe silo 14 in the duct 18 at relatively high velocities as aconsequence of which the recycled powder fractures and new surfaces areexposed to form reaction sites when it is recycled back into the spraydrying chamber 24.

The splitter apparatus 78 for dividing the solid particulates or powderin the storage silo 14 into two streams is shown in greater detail inFIGS. 2 and 3. The splitter apparatus 78 includes a housing 86 whichforms an internal compartment 88, the lower portion of which is dividedinto sections 90 and 92 by means of an internal partial upright wall 94.The splitter compartment 88 receives feed from the silo 14 through theconduit 96 by means of a feeding apparatus such as a rotating dustfeeder 98 illustrated. Inside the compartment 88, the particulate solidscollect in section 90 and overflow wall 94 into section 92 to constituterecycle powder or waste discharge powder, respectively.

The recycle powder in splitter section 90 is fed into a screw feeder 100which feeds the recycle powder into the recycle duct 18 where it ispropelled by the cleaned hot flue gas pumped from the recycle silo fan76 as feed to the spray dryer vessel 12. Powder collected in splittersection 92 is discharged to the discharge line 82 by the rotary dustvalve 80 for collection in a waste storage silo or other disposal.

The rate of splitting is fully adjustable over a wide range by adjustingthe rate of speed of the feed screw 100 through the variable speed drive104. By adjusting this splitting rate, the amount of recycled powder isadjusted. Typically, the amount of recycled powder can be 10 times theamount of flyash and alkali powder that enters the process originally.As a result, the same alkali is caused to pass through the spray dryer12 many times and, thereby, its exposure time to the sulfur dioxide inthe flue gas is greatly increased.

Those skilled in the art, given the teaching herein, will be able todesign the system and process having specific process parameters of gasflow rates, power rates, temperatures, concentrations and equipmenttypes and sizes, depending on the boiler rating, sulfur dioxideconcentration in the flue gas and the emission standards desired to bemet. It is to be understood that such specific process parameters aswell as equipment design characteristics and arrangements are withincontemplation of this invention. For example, although the dustcollection system utilizes filter bags which are preferred becausebaghouse operation and performance are generally less sensitive to inletloading dust, electrostatic precipitation could also be successfullyemployed. Accordingly, there have been disclosed a system and processfor the removal of sulfur dioxide and particulate material from boilerand flue gases utilizing dry scrubbing wherein a portion of thecollected solids are recycled in an efficient and advantageous manner.

What is claimed is:
 1. In a process for removing sulfur dioxide fromflue gas including the steps of feeding the sulfur dioxide-containingflue gas to a spray dryer, contacting the flue gas with an atomizedsolution of alkali reactant in a spray zone in a spray drying chamber toform sulfite and sulfate reaction products, spray drying the solutionand formed materials in the spray drying chamber to obtain dryparticles, conveying the flue gas and dry particles to a dry particlecollector and removing the dry particles in the dry particle collector,the improvement therein comprising recycling a portion of the dryparticles removed in the dry particle collector and contacting thesulfur dioxide containing-flue gas directly with the dry recycledportion of particles.
 2. The improved process as claimed in claim 1wherein the recycled dry particles contact the flue gas in the spraydryer chamber.
 3. The improved process as claimed in claim 1 wherein therecycled dry particles are introduced into the top of the spray dryerchamber and fall vertically downward through the spray zone therein. 4.The improved process as claimed in claim 2 or 3 wherein the alkalireactant is lime.
 5. The improved process as claimed in claim 2 or 3wherein the recycled dry particles are conveyed in such a manner thatthey undergo attrition with creation of fresh particle surface area. 6.The improved process as claimed in claim 1 wherein the dry particlesremoved in the dry collector are conveyed to a storage silo, transportedfrom the storage silo to a splitter apparatus and separated thereby intoa recycle stream and a disposal stream.
 7. A process for dry scrubbingof flue gas containing sulfur dioxide and flyash particles comprisingcontacting the flue gas with recycled gas unreacted alkali reactant andalkali-sulfur dioxide reaction products in a spray dryer, contacting theflue gas with an atomized slurry of alkali reactant in a spray zone inthe spray dryer to react sulfur dioxide with the alkali, conveying theflue gas and dried alkali-sulfur dioxide reaction products, unreactedalkali reactant and flyash to a dust collection system, collecting theparticulate solids, transporting the particulate solids to a splitterapparatus and separating the particulate solids into a recycle streamand a disposal stream, and conveying the particulate solids recyclestream to the spray dryer.